Venous Thrombosis
The development of a blood clot is known as thrombosis. Venous thrombosis (VT) is the formation of a blood clot in the veins. Several conditions can lead to an increased tendency to develop blood clots in the veins or arteries (National Hemophilia Foundation, HemAware newsletter, Vol. 6 (5), 2001), and such conditions may be inherited or acquired. Examples of acquired conditions are surgery and trauma, prolonged immobilization, cancer, myeloproliferative disorders, and even pregnancy, all of which may result in thrombosis (U. Seligsohn and A. Lubetsky, New Eng J Med 344(16):1222-1231, 2001). Inherited causes include mutations in any of several different clotting, anticoagulant, or thrombolytic factors, such as the factor V gene (the factor V Leiden mutation), prothrombin gene (factor II), and methylenetetrahydrofolate reductase gene (MTHFR). Other likely inherited causes are an increase in the expression levels of the factors VIII, IX or XI, or fibrinogen genes (U. Seligsohn and A. Lubetsky, New Eng J Med 344(16):1222-1231, 2001). VT may result from a genetic mutation alone or in concert with environmental factors, such as smoking.
VT is considered a chronic and polygenic disease (A I Schafer, New Engl J Med 340:955-956, 1999). There is evidence to suggest that patients with a first episode of VT be treated with anticoagulant agents (C. Kearon, J A Julian et al., New Engl J Med 340:901-907, 1999).
Venous thrombotic events occur for the first time in about 100 per 100,000 people each year in the United States. About one-third of patients with symptomatic VT manifest pulmonary embolism (PE), whereas two-thirds manifest deep vein thrombosis (DVT) (R H White, Circulation 107(23 Suppl 1):14-8 Review, 2003). DVT is an acute VT in a deep vein, usually in the thigh, legs, or pelvic, and it is a serious and potentially fatal disorder that can arise as a complication for hospital patients, but may also affect otherwise healthy people (A W A Lensing, H R Buller et al., Lancet 353:479-485, 1999). Large blood clots in VT may interfere with blood circulation and impede normal blood flow. In some instances, blood clots may break off and travel to distant major organs such as the brain, heart or lungs as in PE and result in fatality.
VT is a multifactorial disease resulting from both acquired and genetic factors (F R Rosendaal, Lancet, 1999, 353:1167-1173; M D Silverstein, J A Heit et al., Arch Intern Med, 1998; 158:585-593). Over 200,000 new cases of VT occur annually. Of these, 30 percent of patients die within three days; one in five suffer sudden death due to PE (Seminars in Thrombosis and Hemostasis, 2002, Vol. 28, Suppl. 2). Caucasians and African-Americans have a significantly higher incidence than Hispanics, Asians or Pacific Islanders (R H White, Circulation 107(23 Suppl 1):14-8 Review, 2003).
Thus, there is an urgent need for novel genetic markers that are predictive of predisposition to VT, particularly for individuals who are unrecognized as having a predisposition to developing the disease. Such genetic markers may enable screening of VT in much larger populations compared with the populations that can currently be evaluated by using existing risk factors and biomarkers. The availability of a genetic test may allow, for example, appropriate preventive treatments for acute venous thrombotic events to be provided for high risk individuals (such preventive treatments may include, for example, anticoagulant agents). Moreover, the discovery of genetic markers associated with VT may provide novel targets for therapeutic intervention or preventive treatments.
SNPs
The genomes of all organisms undergo spontaneous mutation in the course of their continuing evolution, generating variant forms of progenitor genetic sequences (Gusella, Ann. Rev. Biochem. 55, 831-854 (1986)). A variant form may confer an evolutionary advantage or disadvantage relative to a progenitor form or may be neutral. In some instances, a variant form confers an evolutionary advantage to the species and is eventually incorporated into the DNA of many or most members of the species and effectively becomes the progenitor form. Additionally, the effects of a variant form may be both beneficial and detrimental, depending on the circumstances. For example, a heterozygous sickle cell mutation confers resistance to malaria, but a homozygous sickle cell mutation is usually lethal. In many cases, both progenitor and variant forms survive and co-exist in a species population. The coexistence of multiple forms of a genetic sequence gives rise to genetic polymorphisms, including SNPs.
Approximately 90% of all genetic polymorphisms in the human genome are SNPs. SNPs are single base positions in DNA at which different alleles, or alternative nucleotides, exist in a population. The SNP position (interchangeably referred to herein as SNP, SNP site, SNP locus, SNP marker, or marker) is usually preceded by and followed by highly conserved sequences of the allele (e.g., sequences that vary in less than 1/100 or 1/1000 members of the populations). An individual may be homozygous or heterozygous for an allele at each SNP position. A SNP can, in some instances, be referred to as a “cSNP” to denote that the nucleotide sequence containing the SNP is an amino acid coding sequence.
A SNP may arise from a substitution of one nucleotide for another at the polymorphic site. Substitutions can be transitions or transversions. A transition is the replacement of one purine nucleotide by another purine nucleotide, or one pyrimidine by another pyrimidine. A transversion is the replacement of a purine by a pyrimidine, or vice versa. A SNP may also be a single base insertion or deletion variant referred to as an “indel” (Weber et al., “Human diallelic insertion/deletion polymorphisms,” Am J Hum Genet. 2002 October; 71(4):854-62).
A synonymous codon change, or silent mutation/SNP (terms such as “SNP,” “polymorphism,” “mutation,” “mutant,” “variation,” and “variant” are used herein interchangeably), is one that does not result in a change of amino acid due to the degeneracy of the genetic code. A substitution that changes a codon coding for one amino acid to a codon coding for a different amino acid (i.e., a non-synonymous codon change) is referred to as a missense mutation. A nonsense mutation results in a type of non-synonymous codon change in which a stop codon is formed, thereby leading to premature termination of a polypeptide chain and a truncated protein. A read-through mutation is another type of non-synonymous codon change that causes the destruction of a stop codon, thereby resulting in an extended polypeptide product. While SNPs can be bi-, tri-, or tetra-allelic, the vast majority of the SNPs are bi-allelic, and are thus often referred to as “bi-allelic markers,” or “di-allelic markers.”
As used herein, references to SNPs and SNP genotypes include individual SNPs and/or haplotypes, which are groups of SNPs that are generally inherited together. Haplotypes can have stronger correlations with diseases or other phenotypic effects compared with individual SNPs, and therefore may provide increased diagnostic accuracy in some cases (Stephens et al. Science 293, 489-493, 20 Jul. 2001).
Causative SNPs are those SNPs that produce alterations in gene expression or in the expression, structure, and/or function of a gene product, and therefore are most predictive of a possible clinical phenotype. One such class includes SNPs falling within regions of genes encoding a polypeptide product, i.e. cSNPs. These SNPs may result in an alteration of the amino acid sequence of the polypeptide product (i.e., non-synonymous codon changes) and give rise to the expression of a defective or other variant protein. Furthermore, in the case of nonsense mutations, a SNP may lead to premature termination of a polypeptide product. Such variant products can result in a pathological condition, e.g., genetic disease. Examples of genes in which a SNP within a coding sequence causes a genetic disease include sickle cell anemia and cystic fibrosis.
Causative SNPs do not necessarily have to occur in coding regions; causative SNPs can occur in, for example, any genetic region that can ultimately affect the expression, structure, and/or activity of the protein encoded by a nucleic acid. Such genetic regions include, for example, those involved in transcription, such as SNPs in transcription factor binding domains, SNPs in promoter regions, in areas involved in transcript processing, such as SNPs at intron-exon boundaries that may cause defective splicing, or SNPs in mRNA processing signal sequences such as polyadenylation signal regions. Some SNPs that are not causative SNPs nevertheless are in close association with, and therefore segregate with, a disease-causing sequence. In this situation, the presence of a SNP correlates with the presence of, or predisposition to, or an increased risk in developing the disease. These SNPs, although not causative, are nonetheless also useful for diagnostics, disease predisposition screening, and other uses.
An association study of a SNP and a specific disorder involves determining the presence or frequency of the SNP allele in biological samples from individuals with the disorder of interest, such as VT, and comparing the information to that of controls (i.e., individuals who do not have the disorder; controls may be also referred to as “healthy” or “normal” individuals) who are preferably of similar age and race. The appropriate selection of patients and controls is important to the success of SNP association studies. Therefore, a pool of individuals with well-characterized phenotypes is extremely desirable.
A SNP may be screened in diseased tissue samples or any biological sample obtained from a diseased individual, and compared to control samples, and selected for its increased (or decreased) occurrence in a specific pathological condition, such as pathologies related to VT. Once a statistically significant association is established between one or more SNP(s) and a pathological condition (or other phenotype) of interest, then the region around the SNP can optionally be thoroughly screened to identify the causative genetic locus/sequence(s) (e.g., causative SNP/mutation, gene, regulatory region, etc.) that influences the pathological condition or phenotype. Association studies may be conducted within the general population and are not limited to studies performed on related individuals in affected families (linkage studies).
Clinical trials have shown that patient response to treatment with pharmaceuticals is often heterogeneous. There is a continuing need to improve pharmaceutical agent design and therapy. In that regard, SNPs can be used to identify patients most suited to therapy with particular pharmaceutical agents (this is often termed “pharmacogenomics”). Similarly, SNPs can be used to exclude patients from certain treatment due to the patient's increased likelihood of developing toxic side effects or their likelihood of not responding to the treatment. Pharmacogenomics can also be used in pharmaceutical research to assist the drug development and selection process. (Linder et al. (1997), Clinical Chemistry, 43, 254; Marshall (1997), Nature Biotechnology, 15, 1249; International Patent Application WO 97/40462, Spectra Biomedical; and Schafer et al. (1998), Nature Biotechnology, 16: 3).